Here's a demonstration you can do to start the explanation of why the ball
gets pushed back into the airstream in Balancing Ball:
Hold two pieces of regular 8 1/2 x 11 copier paper, one in each hand, by the
top between your thumb and foreginger, so that the sheets hang. Then position
them so that the flat surfaces of the paper face each other and are about 6
inches apart, with the near edges a few inches in front of your face, and
your mouth is at a level about at the middle of the gap between them (this
would be much easier with a drawing!). Then blow between the two pieces of
paper. You'll notice that the papers come toward each other -- they don't
blow apart, as many people expect.
This is an illustration of the fact that the faster the air moves past a
surface, the lower the pressure. The papers are forced together because the
pressure in the space between them, where the air is moving fast past them,
is lower than that in the space outside the papers, where the air is still.
(This same principle helps explain why roofs are sometimes torn off houses in
tornados. The pressure outside the roof, where air is moving fast, is lower
than the pressure inside, where air is still, and the roof gets pushed upward
off the house. One way to minimize this possibility is to open the windows in
the house so that the higher pressure inside has a greater chance to equalize
with the lower pressure outside the roof.)
The same principle applies to Balancing Ball. As the ball starts to move
outward from the center of the fast-moving air stream, the pressure on the
air-stream side of the ball is lower than the pressure on the other side,
where the air is relatively still. The ball is pushed from an area of high
pressure (still air) to an area of lower pressure (air stream), just like the
pieces of paper. So every time the ball tries to get out of the air stream,
the pressure difference causes it to return.
Hope this helps.
Don Rathjen
Exploratorium Teacher Institute
donr@exploratorium.edu
or
donrath@aol.com
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